The lymphokine, macrophage migration inhibitory factor (MIF), has been identified as a mediator of the function of macrophages in host defense and its expression correlates with delayed hypersensitivity, immunoregulation, inflammation, and cellular immunity. See Metz and Bucala, Adv. Immunol. 66:197-223, 1997. Macrophage migration inhibitory factors (MIFs), which are between 12-13 kilodaltons (kDa) in size, have been identified in several mammalian and avian species; see, for example, Galat et al., Fed. Eur. Biochem. Soc. 319:233-236, 1993; Wistow et al., Proc. Natl. Acad. Sci. USA 90:1272-1275, 1993; Weiser et al., Proc. Natl. Acad. Sci. USA 86:7522-7526, 1989; Bernhagen et al., Nature 365:756-759, 1993; Blocki et al., Protein Science 2:2095-2102, 1993; and Blocki et al., Nature 360:269-270, 1992. Although MIF was first characterized as being able to block macrophage migration, MIF also appears to effect macrophage adherence; induce macrophage to express interleukin-1-beta, interleukin-6, and tumor necrosis factor alpha; up-regulate HLA-DR; increase nitric oxide synthase and nitric oxide concentrations; and activate macrophage to kill Leishmania donovani tumor cells and inhibit Mycoplasma avium growth, by a mechanism different from that effected by interferon-gamma. In addition to its potential role as an immunoevasive molecule, MIF can act as an immunoadjuvant when given with bovine serum albumin or HIV gp120 in incomplete Freunds or liposomes, eliciting antigen induced proliferation comparable to that of complete Freunds. Also, MIF has been described as a glucocorticoid counter regulator and angiogenic factor. As one of the few proteins that is induced and not inhibited by glucocorticoids, it serves to attenuate the immunosuppressive effects of glucocorticoids. As such, it is viewed as a powerful element that regulates the immunosuppressive effects of glucocorticoids. Hence, when its activities/gene expression are overinduced by the administration of excess exogenous glucocorticoids (for example when clinical indicated to suppress inflammation, immunity and the like), there is significant toxicity because MIF itself exacerbates the inflammatory/immune response. See Buccala et al., Ann. Rep. Med. Chem. 33:243-252, 1998.
While MIF is also thought to act on cells through a specific receptor that in turn activates an intracellular cascade that includes erk phosphorylation and MAP kinase and upregulation of matrix metalloproteases, c-jun, c-fos, and IL-1 mRNA (see Onodera et al., J. Biol. Chem. 275:444-450, 2000), it also possesses endogenous enzyme activity as exemplified by its ability to tautomerize the appropriate substrates (e.g., dopachrome). Further, it remains unclear whether this enzymatic activity mediates the biological response to MIF and the activities of this protein in vitro and in vivo. While site directed mutagenesis of MIF has generated mutants which possess full intrinsic activity, yet fail to possess enzyme activity (Hermanowski-Vosatka et al., Biochemistry 38:12841-12849, 1999), Swope et al. have described a direct link between cytokine activity and the catalytic site for MIF (Swope et al., EMBO J. 17(13):3534-3541, 1998). Accordingly, it is unclear that strategies to identify inhibitors of MIF activity through inhibition of dopachrome tautomerase alone yields inhibitors of MIF activity of clinical value. The ability to evaluate the inhibition of MIF to its cell surface receptor is also limited since no high affinity receptor is currently known.
The interest in developing MIF inhibitors derives from the observation that MIF is known for its cytokine activity concentrating macrophages at sites of infection, and cell-mediated immunity. Moreover, MIF is known as a mediator of macrophage adherence, phagocytosis, and tumoricidal activity. See Weiser et al., J. Immunol. 147:2006-2011, 1991. Hence, the inhibition of MIF results in the indirect inhibition of cytokines, growth factors, chemokines and lymphokines that the macrophage may otherwise bring to a site of inflammation. Human MIF cDNA has been isolated from a T-cell line, and encodes a protein having a molecular mass of about 12.4 kDa with 115 amino acid residues that form a homotrimer as the active form (Weiser et al., Proc. Natl. Acad. Sci. USA 86:7522-7526, 1989). While MIF was originally observed in activated T-cells, it has now been reported in a variety of tissues including the liver, lung, eye lens, ovary, brain, heart, spleen, kidney, muscle, and others. See Takahashi et al., Microbiol Immunol. 43(1):61-67, 1999. Another characteristic of MIF is its lack of a traditional leader sequence (i.e., a leaderless protein) to direct classical secretion through the ER/Golgi pathway.
A MIF inhibitor (and a method to identify MIF inhibitors) that act by neutralizing the cytokine activity of MIF presents significant advantages over other types of inhibitors. For example, the link between tautomerase activity alone and the inflammatory response is controversial. Furthermore, inhibitors that act intracellularly are often toxic by virtue of their action on related targets or the activities of the target inside cells. Small molecule inhibitors of the ligand receptor complex are difficult to identify let alone optimize and develop. The ideal inhibitor of a cytokine like MIF is one that alters MIF itself so that when released from the cell it is effectively neutralized. A small molecule with this activity is superior to antibodies because of the fundamental difference between proteins and chemicals as drugs.